Soft magnetic materials are used for applications, such as core materials in inductors, stators and rotors for electrical machines, actuators, sensors and transformer cores. Traditionally, soft magnetic cores, such as rotors and stators in electric machines, are made of stacked steel laminates. Soft Magnetic Composite, SMC, materials are based on soft magnetic particles, usually iron-based, with an electrically insulating coating on each particle. By compacting the insulated particles optionally together with lubricants and/or binders using the traditionally powder metallurgy process, the SMC parts are obtained. By using this powder metallurgical technique it is possible to produce materials having a higher degree of freedom in the design of the SMC component than by using the steel laminates as the SMC material can carry a three dimensional magnetic flux and as three dimensional shapes can be obtained by the compaction process.
Two key characteristics of an iron core component are its magnetic permeability and core loss characteristics. The magnetic permeability of a material is an indication of its ability to become magnetised or its ability to carry a magnetic flux. Permeability is defined as the ratio of the induced magnetic flux to the magnetising force or field intensity. When a magnetic material is exposed to a varying field, energy losses occur due to both hysteresis losses and eddy current losses. The hysteresis loss is brought about by the necessary expenditure of energy to overcome the retained magnetic forces within the iron core component. The eddy current loss is brought about by the production of electric currents in the iron core component due to the changing flux caused by alternating current (AC) conditions.
Research in the powder-metallurgical manufacture of magnetic core components using coated iron-based powders has been directed to the development of iron powder compositions that enhance certain physical and magnetic properties without detrimentally affecting other properties of the final component. Desired component properties include e.g. a high permeability through an extended frequency range, low core losses, high saturation induction, and high strength. Normally an increased density of the component enhances all of these properties. The desired powder properties include suitability for compression moulding techniques, which i.e. means that the powder can be easily moulded to a high density component, which can be easily ejected from the moulding equipment. In order to minimize the eddy current losses in components made of soft magnetic composite powders much effort have been directed to increase the resisitvity of the coating surrounding the soft magnetic metal powder. By altering for example the chemical composition of the coating or the thickness of the coating the resisitvity is affected. However, improvements of the resisitivity normally has a negative effect on the magnetic permeability of a soft magnetic composite component at a given density.
A large number of patent publications teach different types of electrically insulating coatings. Examples of recently published patents concerning inorganic coatings are the U.S. Pat. No. 6,309,748 and U.S. Pat. No. 6,348,265. Coatings of organic materials are known from e.g. the U.S. Pat. No. 5,595,609. Coatings comprising both inorganic and organic material are known from e.g. the U.S. Pat. Nos. 6,372,348 and 5,063,011, according to which publication the particles are surrounded by an iron phosphate layer and a thermoplastic material.
In contrast to the above patents which disclose improvements in one or more properties of the obtained soft magnetic components due to different types of electrical insulation coatings, the present invention is based on the discovery that unexpected advantages can be obtained depending on the nature of the base powder i.e. the powder, the particles of which are not coated or electrically insulated. Especially unexpected is the finding that a more pure base powder increases the resistivity (decreases the eddy current loss) of the final soft magnetic component. It has thus been found that the permeability and total loss can be remarkably improved by using as a base powder a powder which is very pure, has a low oxygen content and a low specific surface.